6,293 research outputs found
Large-scale simulation of steady and time-dependent active suspensions with the force-coupling method
We present a new development of the force-coupling method (FCM) to address
the accurate simulation of a large number of interacting micro-swimmers. Our
approach is based on the squirmer model, which we adapt to the FCM framework,
resulting in a method that is suitable for simulating semi-dilute squirmer
suspensions. Other effects, such as steric interactions, are considered with
our model. We test our method by comparing the velocity field around a single
squirmer and the pairwise interactions between two squirmers with exact
solutions to the Stokes equations and results given by other numerical methods.
We also illustrate our method's ability to describe spheroidal swimmer shapes
and biologically-relevant time-dependent swimming gaits. We detail the
numerical algorithm used to compute the hydrodynamic coupling between a large
collection () of micro-swimmers. Using this methodology, we
investigate the emergence of polar order in a suspension of squirmers and show
that for large domains, both the steady-state polar order parameter and the
growth rate of instability are independent of system size. These results
demonstrate the effectiveness of our approach to achieve near continuum-level
results, allowing for better comparison with experimental measurements while
complementing and informing continuum models.Comment: 37 pages, 21 figure
Angular Momentum and Vortex Formation in Bose-Einstein-Condensed Cold Dark Matter Haloes
(Abridged) Extensions of the standard model of particle physics predict very
light bosons, ranging from about 10^{-5} eV for the QCD axion to 10^{-33} eV
for ultra-light particles, which could be the cold dark matter (CDM) in the
Universe. If so, their phase-space density must be high enough to form a
Bose-Einstein condensate (BEC). The fluid-like nature of BEC-CDM dynamics
differs from that of standard collisionless CDM (sCDM), so observations of
galactic haloes may distinguish them. sCDM has problems with galaxy
observations on small scales, which BEC-CDM may overcome for a large range of
particle mass m and self-interaction strength g. For quantum-coherence on
galactic scales of radius R and mass M, either the de-Broglie wavelength
lambda_deB ~ m_H \cong 10^{-25}(R/100 kpc)^{-1/2}(M/10^{12}
M_solar)^{-1/2} eV, or else lambda_deB << R but self-interaction balances
gravity, requiring m >> m_H and g >> g_H \cong 2 x 10^{-64} (R/100
kpc)(M/10^{12} M_solar)^{-1} eV cm^3. Here we study the largely-neglected
effects of angular momentum. Spin parameters lambda \cong 0.05 are expected
from tidal-torquing by large-scale structure, just as for sCDM. Since lab BECs
develop quantum vortices if rotated rapidly enough, we ask if this angular
momentum is sufficient to form vortices in BEC haloes, affecting their
structure with potentially observable consequences. The minimum angular
momentum for this, L_{QM} = , requires m >= 9.5 m_H for lambda =
0.05, close to the particle mass required to influence structure on galactic
scales. We study the equilibrium of self-gravitating, rotating BEC haloes which
satisfy the Gross-Pitaevskii-Poisson equations, to calculate if and when
vortices are energetically favoured. Vortices form as long as self-interaction
is strong enough, which includes a large part of the range of m and g of
interest for BEC-CDM haloes.Comment: Several typos and numerical typos (incl. in Fig.6, Table 2 and Table
3) have been corrected and references have been updated after proof-reading
stage; MNRAS in press; 29 pages; 11 figure
Long-range Acoustic Interactions in Insect Swarms: An Adaptive Gravity Model
The collective motion of groups of animals emerges from the net effect of the
interactions between individual members of the group. In many cases, such as
birds, fish, or ungulates, these interactions are mediated by sensory stimuli
that predominantly arise from nearby neighbors. But not all stimuli in animal
groups are short range. Here, we consider mating swarms of midges, which
interact primarily via long-range acoustic stimuli. We exploit the similarity
in form between the decay of acoustic and gravitational sources to build a
model for swarm behavior. By accounting for the adaptive nature of the midges'
acoustic sensing, we show that our "adaptive gravity" model makes mean-field
predictions that agree well with experimental observations of laboratory
swarms. Our results highlight the role of sensory mechanisms and interaction
range in collective animal behavior. The adaptive interactions that we present
here open a new class of equations of motion, which may appear in other
biological contexts.Comment: 25 pages, 15 figure
Cosmological Simulations with Self-Interacting Dark Matter II: Halo Shapes vs. Observations
If dark matter has a large self-interaction scattering cross section, then
interactions among dark-matter particles will drive galaxy and cluster halos to
become spherical in their centers. Work in the past has used this effect to
rule out velocity-independent, elastic cross sections larger than sigma/m ~
0.02 cm^2/g based on comparisons to the shapes of galaxy cluster lensing
potentials and X-ray isophotes. In this paper, we use cosmological simulations
to show that these constraints were off by more than an order of magnitude
because (a) they did not properly account for the fact that the observed
ellipticity gets contributions from the triaxial mass distribution outside the
core set by scatterings, (b) the scatter in axis ratios is large and (c) the
core region retains more of its triaxial nature than estimated before.
Including these effects properly shows that the same observations now allow
dark matter self-interaction cross sections at least as large as sigma/m = 0.1
cm^2/g. We show that constraints on self-interacting dark matter from
strong-lensing clusters are likely to improve significantly in the near future,
but possibly more via central densities and core sizes than halo shapes.Comment: 17 pages, 11 figure
Irreversibility and Chaos in Active Particle Suspensions
Active matter has been the object of huge amount of research in recent years
for its important fundamental and applicative properties. In this paper we
investigate active suspensions of micro-swimmers through direct numerical
simulation, so that no approximation is made at the continuous level other than
the numerical one. We consider both pusher and puller organisms, with a
spherical or ellipsoidal shape. We analyse the velocity and the characteristic
scales for an homogeneous two-dimensional suspension and the effective
viscosity under shear. We bring evidences that the complex features displayed
are related to a spontaneous breaking of the time-reversal symmetry. We show
that chaos is not a key ingredient, whereas a large enough number of
interacting particles and a non-spherical shape are needed to break the
symmetry and are therefore at the basis of the phenomenology. Our numerical
study also shows that pullers display some collective motion, though with
different characteristics from pushers
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